Liquid ejecting head drive method and liquid ejection device

ABSTRACT

A liquid-ejecting apparatus according to which targeted good characteristics can be obtained, and moreover the scope for material selection can be broadened is provided. The liquid-ejecting apparatus contracts a pressure chamber and thus ejects liquid through application of voltage to a piezoelectric body, and is such that the driving waveform applied to the piezoelectric body during the liquid ejecting operation comprises a high potential period (a 2 ) in which a voltage exhibiting an electric field strength exceeding the coercive electric field of the piezoelectric body is applied, and a reverse potential period (a 6 ) in which a voltage such that the potential becomes of the opposite polarity to the polarity in the high potential period or the potential becomes zero is applied.

TECHNICAL FIELD

The present invention relates to a liquid-ejecting apparatus that ejectsliquid such as ink through control of the voltage applied topiezoelectric elements, and in particular to a liquid-ejecting apparatussuch as a printer that applies a waveform that enables the piezoelectriccharacteristics of the piezoelectric elements to be exhibited to thefull during a printing operation. Moreover, the present inventionrelates to a liquid-ejecting head driving method in which such awaveform is applied.

BACKGROUND ART

An ink jet recording head comprises pressure chambers that generate inkpressure using piezoelectric elements or heat-generating elements, anink chamber that supplies ink to the pressure chambers, and nozzles thateject ink from the pressure chambers. The pressure is generated byapplying driving signals to the elements in accordance with printingsignals, whereby ink drops are made to fly out from the nozzles onto arecording medium. In particular, with an ink jet recording head thatuses piezoelectric elements, heat is not used, and hence there areadvantages such as degradation of the ink being not prone to occurring,and clogging being not prone to occurring.

With such an ink jet recording head that uses piezoelectric elements,with an aim of improving the ink ejecting characteristics given by thepiezoelectric films, efforts have been made to obtain goodcharacteristics by making the piezoelectric films have a particularcomposition, crystal orientation or the like. For example, the inventorshave discovered that PZT having a 100 plane orientation degree of 70% ormore exhibits good characteristics.

However, it is not easy to manufacture piezoelectric films having aparticular crystal orientation or the like. Moreover, if the crystalorientation or the like is limited to being a particular one, then theflexibility of material selection is narrowed. Consequently, if it werepossible to obtain targeted good characteristics even with, for example,PZT having a 100 plane orientation degree of less than 70%, then theeffects of this would be great.

It is thus an object of the present invention to provide aliquid-ejecting apparatus according to which targeted goodcharacteristics can be obtained, and moreover the scope for materialselection can be broadened.

DISCLOSURE OF THE INVENTION

A liquid-ejecting apparatus according to the present invention is aliquid-ejecting apparatus that contracts a pressure chamber and thusejects liquid through application of voltage to a piezoelectric body,and is such that a driving waveform applied to the piezoelectric bodyduring the liquid ejecting operation comprises a high potential periodin which a voltage exhibiting an electric field strength exceeding thecoercive electric field of the piezoelectric body is applied, and areverse potential period in which a voltage such that the potentialbecomes of the opposite polarity to the polarity in the high potentialperiod or the potential becomes zero is applied. As a result, thecharacteristics of the piezoelectric body can be better exhibited.

In the liquid-ejecting apparatus described above, it is preferable forthe voltage applied in the reverse potential period to be a voltageexhibiting an electric field strength that does not exceed the coerciveelectric field of the piezoelectric body. As a result, thecharacteristics of the piezoelectric body can be exhibited to the full.

In the liquid-ejecting apparatus described above, the voltage applied inthe reverse potential period may be a voltage exhibiting an electricfield strength of at least the coercive electric field of thepiezoelectric body. As a result, residual polarization in thepiezoelectric body can be controlled. In this liquid-ejecting apparatus,it is preferable for the voltage applied in the reverse potential periodto be a voltage that eliminates residual polarization of thepiezoelectric body. Moreover, after the pressure chamber has expanded inthe reverse potential period, when contraction of the pressure chamberhas started, it is preferable for the pressure chamber to be furthercontracted to eject the liquid through the high potential period whilethe pressure chamber has not yet expanded again. As a result, thecontraction of the pressure chamber due to the coercive electric fieldbeing exceeded and the contraction of the pressure chamber due to movinginto the high potential period can be synchronized, and hence thedisplacement amount and the displacement velocity can be increased.Moreover, it is preferable for the time period for which the voltageexhibiting an electric field strength of at least the coercive electricfield is applied to be not more than 2 μs out of the reverse potentialperiod. As a result, the meniscus can be prevented from becomingunstable, and moreover a larger displacement can be obtained. Moreover,it is preferable for the time period between when the absolute value ofthe voltage applied in the reverse potential period starts to drop froma maximum value and when the absolute value of the voltage applied inthe high potential period reaches approximately a maximum to be not morethan 2 μs. As a result, the displacement amount and the displacementvelocity can be increased effectively.

In the liquid-ejecting apparatus described above, it is preferable forthe absolute value of the voltage applied in the reverse potentialperiod to be not more than the absolute value of the maximum voltage inthe high potential period.

In the liquid-ejecting apparatus described above, it is preferable forthe voltage to be applied to a piezoelectric thin film. In particular,it is preferable for a voltage exhibiting an electric field strength ofat least 1.5×10⁷ V/m to be applied in the high potential period.Moreover, it is preferable for the voltage to be applied at a frequencyof at least 20 kHz.

In the liquid-ejecting apparatus described above, it is preferable forthe driving waveform to have one of the reverse potential period per oneof the high potential period.

In the liquid-ejecting apparatus described above, it is preferable for aportion, out of the driving waveform applied during the liquid ejectingoperation, corresponding to during a contraction operation of thepressure chamber to contain at least part of the high potential period,and at least part of the reverse potential period. As a result, a largedisplacement can be used in contracting the pressure chamber.

Moreover, in the liquid-ejecting apparatus described above, it ispreferable for the strain in the piezoelectric body during liquidejection by the liquid-ejecting head to be at least 0.3%.

In the liquid-ejecting apparatus described above, it is preferable forthe driving waveform to be constituted such that the pressure chamber iscontracted and hence liquid is ejected through a change in the potentialfrom a prescribed medium potential to the maximum potential in the highpotential period, and the potential returns to the prescribed mediumpotential via the reverse potential period, and moreover for the changein the potential from the reverse potential period to the prescribedmedium potential to be made to have a gradient such that liquid is notejected.

In the liquid-ejecting apparatus described above, it is preferable forthe potential to be changed continuously from the maximum potential inthe high potential period to the potential in the reverse potentialperiod. As a result, driving at high frequency can be made possible.

In the liquid-ejecting apparatus described above, it may be made to besuch that the voltage of the reverse potential period can be appliedselectively per one application of the voltage of the high potentialperiod. As a result, the size of the liquid drops can be varied.

A driving method of the present invention is a liquid-ejecting headdriving method in which a pressure chamber is contracted and thus liquidis ejected through application of voltage to a piezoelectric body, andis such that a driving waveform applied to the piezoelectric body duringa printing operation comprises a high potential period in which avoltage exhibiting an electric field strength exceeding the coerciveelectric field of the piezoelectric body is applied, and a reversepotential period in which a voltage such that the potential becomes ofthe opposite polarity to the polarity in the high potential period orthe potential becomes zero is applied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view for explaining the structure of a printer,which is a liquid-ejecting apparatus according to an embodiment of thepresent invention.

FIG. 2 is a block diagram showing the electrical constitution of theabove-mentioned printer.

FIG. 3 is an explanatory view of the structure of an ink jet recordinghead used in the above-mentioned printer.

FIG. 4 is a sectional view for explaining the structure of theabove-mentioned ink jet recording head in more detail.

FIG. 5 is a diagram showing the electrical constitution of theabove-mentioned ink jet recording head.

FIG. 6 is a diagram for explaining a procedure for applying drivingpulses to a piezoelectric element appearing in FIG. 5.

FIG. 7 is a waveform diagram of driving waveforms according toliquid-ejecting apparatuses and driving methods of a first example ofthe present invention and a comparative example.

FIG. 8 is a graph showing results of measurements of displacement amountof a piezoelectric thin-film element using the driving waveforms of thefirst example and the comparative example.

FIG. 9 are graphs showing characteristics of strain (S) versus electricfield strength (E) for a piezoelectric thin film.

Regarding FIG. 10, FIG. 10A is a waveform diagram showing an example ofa voltage waveform applied to a piezoelectric element by liquid-ejectingapparatuses of a second example and a fourth example, and FIG. 10B is awaveform diagram showing an example of a voltage waveform applied to apiezoelectric element by liquid-ejecting apparatuses of a third exampleand a fifth example.

FIG. 11 is a graph showing driving waveforms according to a sixthexample and a seventh example.

FIG. 12 is a graph showing the change over time in the displacement inthe case of driving a piezoelectric thin film using the drivingwaveforms of the sixth example and the seventh example.

FIG. 13 is a graph showing the change over time in the displacementvelocity of the diaphragm in the case of carrying out driving using thedriving waveforms of the sixth example and the seventh example.

FIG. 14 are graphs showing an example of the driving signal and thedisplacement of a piezoelectric element according to an eighth exampleof the present invention.

FIG. 15 is a graph showing an example of the driving signal according toa ninth example of the present invention.

FIG. 16 are graphs showing an example of the driving signal and thedisplacement of a piezoelectric element according to a tenth example ofthe present invention.

Note that in the drawings, 10 is a nozzle plate, 20 is a pressurechamber substrate, 30 is a diaphragm, 31 is an insulating film, 32 is abottom electrode, 40 is a piezoelectric element, 41 is a piezoelectricthin-film layer, 42 is an top electrode, and 21 is a pressure chamber.

BEST MODE FOR CARRYING OUT THE INVENTION

Following is a description of embodiments of the present invention, withreference to the drawings.

1. Overall Constitution of Ink Jet Printer

FIG. 1 is a perspective view for explaining the structure of a printer,which is a liquid-ejecting apparatus according to an embodiment of thepresent invention. In the printer, a tray 3, a discharge opening 4 andoperation buttons 9 are provided on/in a main body 2. Furthermore,inside the main body 2 are provided an ink jet recording head 1, whichis a liquid-ejecting head, a paper-feeding mechanism 6, and a controlcircuitry 8.

The ink jet recording head 1 has piezoelectric elements, which will bedescribed later. The ink jet recording head 1 is constituted such thatliquid such as ink can be ejected from nozzles in accordance withejection signals supplied from the control circuitry 8.

The main body 2 is the casing of the printer; the paper-feedingmechanism 6 is disposed in a position so as to be able to feed in paper5 from the tray 3, and the ink jet recording head 1 is disposed so as tobe able to carry out printing on the paper 5. The tray 3 is constitutedsuch that the paper 5 can be fed in to the paper-feeding mechanism 6before printing, and the discharge opening 4 is an outlet from which thepaper 5 is discharged after the printing has been completed.

The paper-feeding mechanism 6 comprises a motor 600, rollers 601 and602, and other mechanical structure that is not shown in FIG. 1. Themotor 600 is able to rotate in accordance with driving signals suppliedfrom the control circuitry 8. The mechanical structure is constituted soas to be able to transmit the rotational power of the motor 600 to therollers 601 and 602. The rollers 601 and 602 are such as to rotate uponthe rotational power of the motor 600 being transmitted thereto, andthrough this rotation draw in paper 5 that has been loaded into the tray3, and feed in the paper 5 so that printing can be carried out by thehead 1.

The control circuitry 8 comprises a CPU, a ROM, a RAM, interfacecircuitry and so on, which are not shown in FIG. 1; the controlcircuitry 8 is such as to be able to supply driving signals to thepaper-feeding mechanism 6 and ejection signals to the ink jet recordinghead 1, this being in accordance with printing data supplied from acomputer via a connector, which is not shown in FIG. 1. Moreover, thecontrol circuitry 8 is such as to be able to carry out operation modesetting, resetting and so on in accordance with operating signals fromthe operation panel 9.

2. Electrical Constitution of Ink Jet Printer

FIG. 2 is a block diagram showing the electrical constitution of theprinter described above. As shown in FIG. 2, the electrical constitutionof the printer in the present embodiment comprises the control circuitry8 and a print engine 12.

The control circuitry 8 comprises an external interface 13 (hereinafterreferred to as the ‘external I/F 13’), a RAM 14 that stores various datatemporarily, a ROM 15 that stores a control program and so on, a controlunit 16 that contains a CPU and so on, an oscillator circuit 17 thatgenerates clock signals, a driving signal generating circuit 19, whichis driving means that generates driving signals to be supplied to theink jet recording head 1, and an internal interface 18 (hereinafterreferred to as the ‘internal I/F 18’) that sends to the print engine 12the driving signals and dot pattern data (bit map data) that has beencreated through expansion based on the printing data.

The external I/F 13 receives, from a host computer or the like, which isnot shown in the drawings, printing data that is constituted from, forexample, character codes, graphics functions, image data, or the like.Moreover, busy signals (BUSY) and acknowledge signals (ACK) areoutputted to the host computer or the like via the external I/F 13.

The RAM 14 functions as a receiver buffer 141, an intermediate buffer142, an output buffer 143, and a working memory, which is not shown inFIG. 2. The receiver buffer 141 temporarily stores printing data thathas been received by the external I/F 13, the intermediate buffer 142stores intermediate code data that has been created through conversionby the control unit 16, and the output buffer 143 stores dot patterndata. This dot pattern data is constituted from printing data obtainedby decoding (translating) gradation data.

Moreover, in addition to the control program (control routines), whichis for carrying out various types of data processing, the ROM 15 alsostores font data, graphics functions, and so on.

The control unit 16 reads out printing data from the receiver buffer141, and also stores intermediate code data obtained by converting thisprinting data into the intermediate buffer 142. Moreover, the controlunit 16 analyzes intermediate code data read out from the intermediatebuffer 142, and referring to the font data, graphics functions and so onstored in the ROM 15, expands the intermediate code data into dotpattern data. The control unit 16 then carries out required embellishingprocessing, and then stores the dot pattern data that has been createdthrough expansion into the output buffer 143.

When the dot pattern data corresponding to one line's worth for the inkjet recording head 1 has been obtained, this one line's worth of dotpattern data is outputted to the ink jet recording head 1 via theinternal I/F 18. Moreover, once this one line's worth of dot patterndata has been outputted from the output buffer 143, the intermediatecode data for which the expansion has been completed is deleted from theintermediate buffer 142, and then expansion processing is carried out onthe next batch of intermediate code data.

The print engine 12 comprises the ink jet recording head 1, thepaper-feeding mechanism 6, and a carriage mechanism 7.

The paper-feeding mechanism 6 is constituted from the paper-feedingmotor, the paper-feeding rollers and so on, and progressively feedsthrough a printing recording medium such as recording paper insynchronization with the recording operation of the ink jet recordinghead 1. That is, the paper-feeding mechanism 6 moves the printingrecording medium relatively in a secondary scanning direction.

The carriage mechanism 7 is constituted from a carriage main body onwhich the ink jet recording head 1 can be mounted, and a carriagedriving unit that makes the carriage main body travel along a principalscanning direction. By making the carriage main body travel, the ink jetrecording head 1 can be moved in the principal scanning direction. Notethat for the carriage driving unit, any mechanism can be adopted so longas it is a mechanism that enables the carriage main body to be made totravel, for example a carriage driving unit that uses a timing belt canalso be used.

The ink jet recording head 1 has a large number of nozzles along thesecondary scanning direction, and ejects ink drops from the nozzles witha timing governed by the dot pattern data or the like.

3. Constitution of Ink Jet Recording Head

FIG. 3 is an explanatory view of the structure of the ink jet recordinghead used in the printer, or liquid-ejecting apparatus, described above.The ink jet recording head 1 is a so-called flexural oscillation ink jetrecording head, and as shown in the FIG. 3, is constituted comprises anozzle plate 10, a pressure chamber substrate 20, and a diaphragm 30.This head constitutes a piezo jet type head.

The pressure chamber substrate 20 comprises pressure chambers (cavities)21, side walls (partitions) 22, a reservoir 23, and supply openings 24.The pressure chambers 21 are spaces for storing ink or the like forejection and are formed by etching a substrate made of silicon or thelike. The side walls 22 are formed so as to partition the pressurechambers 21 from one another. The reservoir 23 is a common channel forsupplying ink to all of the pressure chambers 21. The supply openings 24are formed so as to enable the ink to be introduced into the pressurechambers 21 from the reservoir 23.

The nozzle plate 10 is stuck onto one surface of the pressure chambersubstrate 20 so that nozzles 11 in the nozzle plate 10 are disposed inpositions corresponding respectively to the pressure chambers 21provided in the pressure chamber substrate 20. The pressure chambersubstrate 20 having the nozzle plate 10 stuck thereon is further putinto a casing 25, thus constituting the ink jet recording head 1.

The diaphragm 30 is stuck onto the other surface of the pressure chambersubstrate 20. Piezoelectric elements (not shown) are provided on thediaphragm 30. An ink tank connection port (not shown) is provided in thediaphragm 30, whereby ink stored in an ink tank, not shown, can besupplied to the reservoir 23 in the pressure chamber substrate 20.

4. Layer Structure

FIG. 4 is a sectional view for explaining the structure of the ink jetrecording head described above in more detail. This sectional view is anenlargement of a section through one pressure chamber and onepiezoelectric element. As shown in FIG. 4, the diaphragm 30 isconstituted from an insulating film 31 and a bottom electrode 32laminated together, and the piezoelectric element 40 is constituted froma piezoelectric thin-film layer 41 and a top electrode 42 laminated onthe bottom electrode 32. The ink jet recording head 1 is constitutedsuch that the piezoelectric elements 40, the pressure chambers 21 andthe nozzles 11 are provided in a line with a constant pitch. Thisinter-nozzle pitch can be subjected to design modification as requiredin accordance with the printing precision. For example, the nozzles maybe provided at 400 dpi (dots per inch).

The insulating film 31 is formed to a thickness of approximately 1 μmfrom a material that is not electrically conductive, for example silicondioxide (SiO₂), and is constituted so as to be able to deform upondeformation of the piezoelectric thin-film layer, whereby the pressureinside the pressure chamber 21 can be increased momentarily.

The bottom electrode 32 is one of the electrodes for applying a voltageto the piezoelectric thin-film layer, and is formed to a thickness ofapproximately 0.2 μm from a material that is electrically conductive,for example platinum (Pt) or the like. The bottom electrode 32 is formedin the same region as the insulating film 31 so as to function as acommon electrode for the plurality of piezoelectric elements formed onthe pressure chamber substrate 20. Note, however, that it is alsopossible to form bottom electrodes to the same size as the piezoelectricthin-film layers 41, i.e. in the same shape as the top electrodes.

Each top electrode 42 is the other electrode for applying a voltage tothe corresponding piezoelectric thin-film layer, and is formed to athickness of approximately 0.1 μm from a material that is electricallyconductive, for example platinum (Pt) or iridium (Ir).

Each piezoelectric thin-film layer 41 comprises a crystal of apiezoelectric ceramic such as lead zirconate titanate (PZT) having aperovskite structure, and is formed on the diaphragm 30 in a prescribedshape. Each piezoelectric thin-film layer 41 is formed to a thickness ofpreferably not more than 2 μm, for example approximately 1 ∥m. Thecoercive electric field of such a piezoelectric thin film is, forexample, approximately 2×10⁶ V/m.

5. Printing Operation

A description will now be given of the printing operation for the inkjet recording head 1 having the constitution described above. Drivingsignals are outputted from the control circuitry 8, whereby thepaper-feeding mechanism 6 operates and paper 5 is thus conveyed in to aposition such that printing can be carried out by the head 1. If anejection signal is not supplied from the control circuitry 8 and hence avoltage is not applied between the bottom electrode 32 and the topelectrode 42 of a piezoelectric element 40, no deformation occurs in thepiezoelectric thin-film layer 41. A pressure change thus does not occurin a pressure chamber 21 on which is provided a piezoelectric element 40to which an ejection signal is not supplied, and hence an ink drop isnot ejected from the nozzle 11 of that pressure chamber 21.

On the other hand, if an ejection signal is supplied from the controlcircuitry 8 and hence a certain voltage is applied between the bottomelectrode 32 and the top electrode 42 of a piezoelectric element 40,deformation occurs in the piezoelectric thin-film layer 41. Thediaphragm 30 flexes greatly at a pressure chamber 21 on which isprovided a piezoelectric element 40 to which an ejection signal has beensupplied. The pressure inside the pressure chamber 21 thus risesmomentarily, and hence an ink drop is ejected from the nozzle 11. Byindividually supplying ejection signals to piezoelectric elements inpositions in the head corresponding to the printing data, characters andgraphics can be printed as desired.

6. Electrical Constitution of Ink Jet Recording Head

Next, a more detailed description will be given of the electricalconstitution of the ink jet recording head described above, withreference to FIG. 5.

As shown in FIG. 5, the ink jet recording head 1 has shift registers 51,latch circuitry 52, level shifters 53, switches 54, the piezoelectricelements 40, and so on. Furthermore, as shown in FIG. 5, the shiftregisters 51, latch circuitry 52, level shifters 53, switches 54 andpiezoelectric elements 40 are constituted respectively from shiftregister elements 51A to 51N, latch elements 52A to 52N, level shifterelements 53A to 53N, switch elements 54A to 54N, and piezoelectricelements 40A to 40N, which are provided for the respective nozzles 11 ofthe ink jet recording head 1. The shift registers 51, latch circuitry52, level shifters 53, switches 54 and piezoelectric elements 40 areconnected together electrically in this order.

The shift registers 51, latch circuitry 52, level shifters 53 andswitches 54 produce driving pulses from ejection driving signalsgenerated by the driving signal generating circuit 19. Here, the drivingpulses are applied pulses that are actually applied to the piezoelectricelements 40.

FIG. 6 is a diagram for explaining the procedure for applying drivingpulses (driving signals) to a piezoelectric element. A description willnow be given of the control of the ink jet recording head 1 having anelectrical constitution as described above, with reference to FIG. 6.

With the ink jet recording head 1 having an electrical constitution asdescribed above, as shown in FIG. 6, initially, in synchronization witha clock signal (CK) from the oscillator circuit 17, printing data (SI)constituting dot pattern data is serially transferred from the outputbuffer 143 to the shift registers 51, and is set in order. If the datais set, first, the most significant bit data in the printing data forall of the nozzles 11 is serially transferred. Then, once the serialtransfer of the most significant bit data has been completed, the secondmost significant bit data is serially transferred. Thereafter, the lowerorder bit data is similarly serially transferred in order.

Once the printing data for the bit in question has been set in the shiftregister elements 51A to 51N for all of the nozzles, the control unit 16outputs a latch signal (LAT) to the latch circuitry 52 with a prescribedtiming. Through this latch signal, the latch circuitry 52 latches theprinting data that has been set in the shift registers 51. The printingdata that has been latched by the latch circuitry 52 (LATout) is appliedto the level shifters 53, which are voltage amplifiers. In the case thatthe printing data is, for example, ‘1’, the level shifter 53 raises thevoltage of the printing data up to a voltage value such that the switch54 can be driven, for example a few tens of volts. The printing data forwhich the voltage has been raised is then applied to the switch elements54A to 54N, and the switch elements 54A to 54N go into a connected statein accordance with the printing data.

Moreover, an ejection driving signal generated by the driving signalgenerating circuit 19 is also applied to each of the switch elements 54Ato 54N. Consequently, if a switch element 54A to 54N is in a connectedstate, then the ejection driving signal is applied to the piezoelectricelement 40A to 40N connected to that switch element 54A to 54N.

In this way, with the ink jet recording head 1 given as an example here,whether or not the ejection driving signal is applied to each of thepiezoelectric elements 40 can be controlled through the printing data.For example, in a period in which the printing data is ‘1’, the switch54 goes into a connected state through the latch signal (LAT), and hencethe driving signal (COMout) can be supplied to the piezoelectric element40. The piezoelectric element 40 then undergoes displacement(deformation) due to the supplied driving signal (COMout). Moreover, ina period in which the printing data is ‘0’, the switch 54 goes into anunconnected state, and hence supply of the driving signal to thepiezoelectric element 40 is cut off. Note that in such a period in whichthe printing data is ‘0’, the potential from immediately before is heldfor each of the piezoelectric elements 40, and hence the state ofdisplacement from immediately before is maintained.

7. FIRST EXAMPLE AND COMPARATIVE EXAMPLE

FIG. 7 is a waveform diagram of driving waveforms according toliquid-ejecting apparatuses and driving methods of a first example ofthe present invention and a comparative example. FIG. 8 is a graphshowing results of measurements of displacement amount of apiezoelectric thin-film element using these driving waveforms. As shownin FIG. 7, a trapezoidal wave that comprises an 8 μs potential risingperiod, a 20 μs maximum potential maintaining period, and an 8 μspotential falling period, and for which the difference between theminimum potential and the maximum potential is 25V, is used as thedriving waveform. The offset voltage (the DC voltage between the minimumpotential in the driving waveform and the earth potential) ΔV for thistrapezoidal wave was variously changed, piezoelectric thin-film elementswere driven, and the displacement amount was measured. The case that theoffset voltage ΔV is less than zero corresponds to the first example inwhich there is a reverse potential period, and the case that the offsetvoltage ΔV is greater than of equal to zero corresponds to thecomparative example in which there is no reverse potential period. Asthe piezoelectric thin-film elements, measurements were carried out forthree samples using PZT having a (100) orientation degree of 79% (group1), and three samples using PZT having a (100) orientation degree of 33%(group 2), and in each case the average was calculated.

In the case that the group 1 PZT was used, first, a displacement ofapproximately 420 nm to 450 nm was obtained for offset voltage ΔV≧0,which corresponds to the comparative example. If such a displacementamount is obtained, then use as an ink jet recording head is possible,but it is preferable for the displacement amount to be higher. Next,when measurements were carried out with offset voltage ΔV<0, whichcorresponds to the first example, the displacement amount rose, with amaximum displacement of 513 nm being obtained around ΔV=−3V.

In the case that the group 2 PZT was used, first, the displacement wasapproximately 290 nm to 315 nm for offset voltage ΔV≧0, whichcorresponds to the comparative example. This displacement amount is notreally sufficient compared with group 1, and it is preferable for thedisplacement amount to be higher. Next, when measurements were carriedout with offset voltage ΔV<0, which corresponds to the first example,the displacement amount greatly rose, with a maximum displacement of 451nm being obtained around ΔV=−4.3 V.

Note that for both group 1 and group 2, when the offset voltage ΔV wasmade yet lower (i.e. the absolute value made higher), the displacementamount dropped. It is presumed that this is because if the offsetvoltage ΔV is too low then the coercive electric field is exceeded, andhence the flexion inverts.

As described above, for both group 1 and group 2, by using theliquid-ejecting apparatus of the first example, the displacement amountincreased compared with the comparative example. This increase in thedisplacement amount is explained through the hysteresis curve in FIG.9A. As shown in FIG. 9A, with the driving of the comparative example inwhich there is no reverse potential period, the curve becomes as shownby the broken line A, and with the driving of the first example in whichthere is a reverse potential period, the curve becomes as shown by thebroken line B. It can be seen that with the same amount of change in theelectric field strength (E), a larger strain (S) is obtained with thebroken line B.

Furthermore, even with piezoelectric elements such as group 2 for whichit may be considered that sufficient characteristics cannot be obtainedin the case of the comparative example, by using the liquid-ejectingapparatus of the first example, the displacement amount increasesmarkedly, and characteristics sufficient for use can be obtained; thescope for material selection thus increases.

Moreover, in the case that driving was carried out using theliquid-ejecting apparatus according to the comparative example, uponcarrying out driving a large number of times, i.e. for 100 millionpulses or more, the displacement amount dropped by approximately 12%compared with the initial displacement amount, but in the case thatdriving was carried out using the liquid-ejecting apparatus according tothe first example, it was found that the drop in the displacement waskept down to not more than 5% upon carrying out driving a large numberof times. It is presumed that the reason for this is as follows. In thecase that driving is carried out with an electric field higher than thecoercive electric field of the piezoelectric body, if driving is carriedout a large number of times, then the hysteresis curve changes to likethe broken line shown in FIG. 9B. As a result, a drop in thedisplacement occurs with driving in which there is no reverse potentialperiod. However, by providing a reverse potential period, sufficientdisplacement can be obtained even if the hysteresis curve changes.

The optimum value of the offset voltage ΔV for obtaining the maximumdisplacement differs between group 1 and group 2. It is thus preferableto adjust the value of the offset voltage ΔV in accordance with therequired characteristics.

8. SECOND EXAMPLE AND THIRD EXAMPLE

FIG. 10 are waveform diagrams showing examples of a voltage waveformapplied to a piezoelectric element during a printing operation usingliquid-ejecting apparatuses of other examples of the present invention.In particular, FIG. 10A shows one period's worth of the waveform for asecond example, and FIG. 10B shows one period's worth of the waveformfor a third example. When these waveforms are applied to thepiezoelectric thin film, the waveform is applied at a frequency of 20kHz to 50 kHz. This waveform is the waveform applied during the printingoperation, and thus the waveform applied when printing is suspended, forexample during head cleaning or an ink cartridge replacement sequence,may be different to this.

The driving waveform shown in FIG. 10A here comprises a potentialmaintaining period a4, a potential falling period a5, a potentialmaintaining period a6, a potential rising period a1, a potentialmaintaining period a2, and a potential falling period a3.

In the potential maintaining period a4, residual oscillation of themeniscus is stabilized. In the potential falling period a5 and thepotential maintaining period a6, the meniscus is temporarily drawn intothe nozzle, and moreover ink is newly drawn in from the ink tank, notshown in the drawings, thus preparing for ejection in the followingpotential rising period a1. In the potential rising period a1 and thepotential maintaining period a2, a voltage is applied to thepiezoelectric body to contract the pressure chamber, whereby ink isejected from the nozzle. In the potential falling period a3, thepressure chamber is expanded, thus drawing the remaining ink that hasnot been ejected into the nozzle.

In particular, in the potential maintaining period a6, a voltage (−V₁)of having an opposite polarity to that in the potential maintainingperiod a2 when ink is ejected is applied to the piezoelectric body. Byproviding the driving waveform with such a reverse potential period inwhich a voltage such as that in the potential maintaining period a6 isapplied, it becomes possible to exhibit the characteristics of thepiezoelectric body to the full. To exhibit the characteristics of thepiezoelectric body more effectively, it is preferable to provide onereverse potential period in which a voltage such as that in thepotential maintaining period a6 is applied per one ink ejection.

In the potential maintaining period a2, the applied voltage is set suchthat the electric field strength in the piezoelectric body becomes atleast 1.5×10⁷ V/m. For example, the applied voltage in the potentialmaintaining period a2 is set to a value having a high absolute value ofapproximately 20 to 30V. In this case, if the thickness of thepiezoelectric thin film is made to be 1 μm, then the electric fieldstrength in the piezoelectric body during the potential maintainingperiod a2 is 2×10⁷ to 3×10⁷ V/m, which is as high as approximately tentimes the coercive electric field 2×10⁶ V/m of the piezoelectric body inthe present example.

As with the first example, to exhibit the characteristics of thepiezoelectric body effectively, it is preferable for the potential (−V₁)in the reverse potential period including the potential maintainingperiod a6 to be a potential such that the absolute value of the electricfield strength in the piezoelectric body does not exceed the coerciveelectric field of the piezoelectric body. Moreover, it is preferable forthe absolute value of the potential (−V₁) in the reverse potentialperiod including the potential maintaining period a6 to be not more thanthe maximum value of the absolute value of the potential in a highpotential period such as the potential maintaining period a2. Forexample, if the thickness of the piezoelectric body is made to be 1 μm,and the potential (−V₁) in the potential maintaining period a6 is madeto be −2V, then the absolute value of the electric field strength in thepiezoelectric body becomes 2×10⁶ V/m.

In the potential rising period al in which a contraction operation ofthe pressure chamber is carried out, the potential rises from thenegative potential following on from the potential maintaining perioda6, and reaches the maximum potential at the potential maintainingperiod a2.

The driving waveform shown in FIG. 10B comprises parts like theabove-mentioned a1 to a6, and in addition a potential rising period a7,a potential maintaining period a8, a potential falling period a9, and apotential maintaining period a10. The purpose of the potential risingperiod a7, the potential maintaining period a8 and the potential fallingperiod a9 is to control the meniscus for the ink ejection that iscarried out in the potential rising period a1 and the potentialmaintaining period a2; there is an effect of improving the ejectioncharacteristics by giving the meniscus desired oscillation before theink ejection.

As with the waveform of FIG. 10A, it is preferable for the voltage (−V₂)in the potential maintaining period a6 to be a voltage such that theabsolute value of the electric field strength in the piezoelectric bodydoes not exceed the coercive electric field, and is not more than themaximum value of the electric field during ink ejection.

9. FOURTH EXAMPLE AND FIFTH EXAMPLE

In the first to third examples described above, a description was givenof advantages in the case that the electric field strength in thepiezoelectric body during the reverse potential period does not exceedthe coercive electric field, but this electric field strength may exceedthe coercive electric field. Here, the cases that the potential (−V₁ or−V₂) in the potential maintaining period a6 out of the reverse potentialperiod in the driving waveforms of FIG. 10A and FIG. 10B is made to be apotential such that the absolute value of the electric field strength inthe piezoelectric body becomes greater than the coercive electric fieldof the piezoelectric body are taken to be a fourth example and a fifthexample respectively. In such a case, it is preferable for the absolutevalue of the potential (−V₁ or −V₂) in the potential maintaining perioda6 to be not more than the absolute value of the potential in thepotential maintaining period a2. For example, if the thickness of thepiezoelectric body is made to be 1 ∥m, and the potential (−V₁) in thepotential maintaining period a6 is made to be −5V, then the absolutevalue of the electric field strength in the piezoelectric body becomes5×10⁶ V/m.

In this way, by making the potential in the potential maintaining perioda6 be a potential that exhibits an electric field strength exceeding thecoercive electric field, polarization remaining in the piezoelectricfilm during the driving waveform, i.e. at times other than times whenprinting is suspended, can be eliminated. If the piezoelectric body ismade to be a thin film, then the residual polarization tends to droprelatively quickly, and hence even if polarization treatment is carriedout as in Japanese Patent Laid-open No. 9-141866, the polarization dropsif driving is not carried out for a while thereafter. In this case, adifference in polarization arises between elements having a drivinghistory and elements not having a driving history, and hence variationarises between the elements. In the present examples, a voltage of theopposite polarity to the ejection voltage is applied during the drivingwaveform, and hence variation in the displacement between piezoelectricelements can be effectively suppressed even in the case that theprinting operation is continued for a prolonged period.

Moreover, in the case of driving a liquid-ejecting head that usespiezoelectric thin films in particular, the strain in the piezoelectricthin films is high, becoming 0.3% or more. Furthermore, the elasticrestoring force of the substrate cannot be made to be sufficient, andhence residual strain is prone to arising in the piezoelectric thinfilms. Eliminating the residual polarization is thus very important.

10. SIXTH EXAMPLE AND SEVENTH EXAMPLE

FIG. 11 shows driving waveforms of a sixth example and a seventhexample, which are modifications of the fourth example. FIG. 12 showsthe change over time in the displacement in the case of driving apiezoelectric thin film using these driving waveforms. The two drivingwaveforms shown in FIG. 11 have the common feature that the minimumvalue of the voltage applied during the reverse potential period is −5V,but the time period for which this voltage of −5V is applied differs.The waveform shown by the full line (W6) is for the sixth example, andthe time period for which the voltage of −5V is applied is set to 2 μs.On the other hand, the waveform shown by the broken line (W7) is for theseventh example, and the time period for which the voltage of −5V isapplied is set to 0.13 μs.

In the case that the coercive electric field of the piezoelectric thinfilm is made to be 2×10⁶ V/m, and the thickness of the piezoelectricthin film is made to be 1.5 μm, if a voltage lower than −3V is appliedto the piezoelectric thin film, then the electric field strength in thepiezoelectric thin film exceeds the coercive electric field. The timeperiod for which the electric field strength exceeds the coerciveelectric field, i.e. the time period for which a voltage lower than −3Vis applied, is approximately 3 μs in the case of the waveform W6, andapproximately 1.5 μs in the case of the waveform W7.

The curve C6 in FIG. 12 shows the change over time in the displacementin the case that the waveform W6 of FIG. 11 was applied. The differencebetween the maximum value and the minimum value of the displacement was344 nm. As shown by this curve C6, if the coercive electric field isexceeded in the reverse potential period, then the direction of flexioninverts. That is, if the applied voltage is reduced, then thedisplacement drops until the coercive electric field is reached, butafter the coercive electric field has been reached, the displacementrises even if the applied voltage is reduced. This shows that thepolarization has inverted through the coercive electric field beingexceeded. If the direction of flexion of the piezoelectric thin filminverts as in the curve C6, then the movement of the meniscus when theliquid-ejecting head is driven will become unstable, and hence it willbecome difficult to eject drops precisely.

On the other hand, the curve C7 in FIG. 12 shows the change over time inthe displacement in the case that the waveform W7 of FIG. 11 wasapplied. It was found that upon making the time period for which thecoercive electric field is exceeded during the reverse potential periodbe less than 2 μm, the direction of flexion does not reverse during thereverse potential period. Moreover, the difference between the maximumvalue and the minimum value of the displacement was 359 nm, and hence itwas found that the displacement amount also becomes larger compared withthe case of the curve C6.

FIG. 13 is a graph showing the results of measurements of the changeover time in the displacement velocity of the diaphragm in the casesthat a liquid-ejecting head was driven using the driving waveformsdescribed above. The displacement velocity D7 of the diaphragm duringthe contraction operation of the pressure chamber from the reversepotential period to the high potential period in the case of the seventhexample increased to a maximum of around 1 m/s, whereas the displacementvelocity D6 of the diaphragm during the contraction operation in thecase of the sixth example had a maximum of 0.5 m/sec, which isapproximately half of that for the seventh example. It is apparent fromthis that the displacement velocity of the diaphragm can be increased byusing the driving waveform of the seventh example.

As described above, with the driving method of the seventh example, thepressure chamber 21 is expanded by changing the applied voltage in thereverse potential period as far as a potential having an absolute valuehigher than the potential at which a coercive electric field arises inthe piezoelectric thin-film layer 41. Moreover, it has been made to besuch that after contraction of the pressure chamber 21 has started withthe coercive electric field, a high potential period is moved into whileinversion to expansion has still not occurred, and the pressure chamberis further contracted, thus ejecting liquid. As a result, thedisplacement amount and the displacement velocity of the diaphragm 30can be increased. That is, with the driving method of the presentinvention, the displacement of the diaphragm 30 due to the coerciveelectric field is made to act as displacement for during ink ejection,and hence the displacement amount and the displacement velocity of thediaphragm 30 during contraction of the pressure chamber 21 can besubstantially increased.

Moreover, with the present example, if the time period for which thecoercive electric field is exceeded during the reverse potential periodis set to be less than 2 μs, then the displacement amount of thediaphragm 30 due to the coercive electric field can be made to act asdisplacement for during ink ejection. Moreover, if the time periodbetween starting and finishing to contract of the pressure chamber 21 isset to be less than 2 μs, then the transition from the start of thecontraction in the reverse potential period to the end of thecontraction in the high potential period becomes smooth, and hence thedisplacement amount of the diaphragm 30 can be increased effectively. Asa result, there is also an advantage that the ink ejection speed can bemade faster.

11. EIGHTH EXAMPLE AND NINTH EXAMPLE

FIG. 14 are graphs showing an example of the driving signal and thedisplacement of a piezoelectric element according to an eighth exampleof the present invention.

In the eighth example, as shown in FIG. 14A, the basic driving signal(COM) applied to the piezoelectric element 40 has a high potentialperiod 60 and a reverse potential period 70. An ink drop is ejectedthrough the voltage of the high potential period 60 being outputted tothe piezoelectric element 40 in accordance with printing data. Afterthat, the voltage of the reverse potential period 70 is outputted to thepiezoelectric element 40. In the present example, one high potentialperiod 60 and one reverse potential period 70 are outputted alternately.

Here, the ink jet recording head 1 in the present example is a so-called‘draw fire’ type ink jet recording head. The high potential period 60 isconstituted from the following steps: a first expansion step 61 ofreducing the potential from a state in which a medium potential VM ismaintained down to a potential VL, thus expanding the pressure chamber21; a first holding step 62 of maintaining the minimum potential VL fora certain time period; a contraction step 63 of increasing the potentialfrom the minimum potential VL to a maximum potential VH, thuscontracting the pressure chamber 21 and hence ejecting an ink drop; asecond holding step 64 of maintaining the maximum potential VH for acertain time period; and a second expansion step 65 of reducing thepotential from the maximum potential VH to the medium potential VM.

The reverse potential period 70, on the other hand, is constituted fromthe following steps: a reducing step 71 of reducing the potential fromthe medium potential VM to a prescribed potential VR that is zero orbelow; a holding step 72 of maintaining the prescribed potential VR fora certain time period; and an increasing step 73 of increasing thepotential from the prescribed potential VR to the medium potential VM.

When the piezoelectric element 40 is driven using a high potentialperiod 60 as described above, as shown in FIG. 14B, the piezoelectricelement 40 deforms from a medium displacement DM to a minimumdisplacement DL during the first expansion step 61, whereby the meniscusin the nozzle 11 is drawn in toward the pressure chamber 21 side. Next,the contraction step 63 is carried out via the first holding step 62,and hence the piezoelectric element 40 deforms as far as a maximumdisplacement DH, whereby an ink drop is ejected. Specifically, thecontraction step 63 is carried out at a timing when the meniscus ispushed out toward the nozzle 11 side due to the oscillation caused bythe first expansion step 61. As a result, the oscillation of themeniscus due to the first expansion step 61 and the oscillation of themeniscus due to the contraction step 63 are superimposed, and hence theink drop is ejected from the nozzle 11 at a relatively high speed. Afterthat, the displacement of the piezoelectric element 40 is returned tothe original displacement through the second expansion step 65.

Here, in the second expansion step 65, by reducing the potential fromthe maximum potential VH to the medium potential VM, an attempt is madeto return the displacement of the piezoelectric element 40 from themaximum displacement DH to the medium displacement DM as shown by thedashed line in FIG. 14B. However, in actual fact the strain in thepiezoelectric element 40 does not return as far as the mediumdisplacement DM, but rather the displacement of the piezoelectricelement 40 is maintained at a medium displacement DM′.

In the present example, it has thus been made to be such that thepotential is returned to the medium potential VM via the reversepotential period 70 after the high potential period 60, whereby thedisplacement of the piezoelectric element 40 is returned to theprescribed medium displacement DM.

Specifically, after the ink drop ejection, when the potential is reducedto zero or below, for example to −5V, through the reducing step 71 ofthe reverse potential period 70, then the displacement of thepiezoelectric element 40 first changes to a displacement below themedium displacement DM. After that, when the potential is returned tothe medium potential VM through the increasing step 73 via the holdingstep 72, then the displacement of the piezoelectric element 40 returnsto the medium displacement DM. As a result, the displacement amount ofthe piezoelectric element 40 due to the following high potential period60 is stabilized, and hence an ink drop of the desired size can beejected.

Here, the reducing step 71 in the reverse potential period 70 should besuch that the potential can be reduced down to zero or below, and thereis no particular limitation on the gradient of the potential, but it ispreferable to make the gradient in the increasing step 73 relatively lowto the extent that there is no effect on the oscillation of themeniscus. This is because with the ink jet recording head 1 of thepresent example, when the piezoelectric element 40 is driven through theincreasing step 73, the pressure chamber 21 contracts and thus anoscillation arises in the meniscus in a direction of ink drop ejection,and hence if the gradient in the increasing step 73 is made large thenthere will be a risk of an ink drop being accidentally ejected.

Moreover, if the gradient in the increasing step 73 is made to be toolow, then it will be necessary to make the ink drop ejection intervallong and thus high-speed driving will no longer be possible, and henceit is preferable to make the gradient in the increasing step 73 be aslarge as possible but such that there is no affect on the oscillation ofthe meniscus.

In this way, in the present example, it has been made to be such that areverse potential period 70 is provided between each of the highpotential periods 60, and hence when the voltage of the high potentialperiod 60 is outputted to the piezoelectric element 40, the displacementof the piezoelectric element 40 is always maintained at the mediumdisplacement DM. The displacement amount of the piezoelectric element 40due to each high potential period 60 is thus substantially increased.Moreover, even if the maximum potential VH in the high potential period60 is reduced, the current displacement amount is maintained, andmoreover the durability can be improved. Furthermore, the displacementamount of the piezoelectric element 40 due to each high potential period60 is stabilized, and hence printing can be carried out always with thedesired dot size even in the case of driving at a relatively high speed.

In the present example, it was made to be such that after ejection of anink drop through a high potential period 60, there is a prescribed timeinterval before the voltage of the reverse potential period 70 isoutputted, but there is no limitation to this. For example, as with thedriving waveform of a ninth example shown in FIG. 15, the voltage of thereverse potential period 70 may be outputted immediately afteroutputting the voltage of the high potential period 60, i.e. thepotential may be changed continuously from the maximum potential in thehigh potential period to the potential of the reverse potential period.In either case, the potential is temporarily reduced down to zero orbelow, whereby the strain in the piezoelectric element 40 can reliablybe returned to a prescribed medium displacement. Moreover, if the timeinterval between the high potential period 60 and the reverse potentialperiod 70 is made to be short, then printing at relatively high speedcan be carried out.

Moreover, in the eighth example and the ninth example, it was made to besuch that the gradient in the increasing step 73 of the reversepotential period 70 is made to be low, whereby accidental ejection of anink drop while returning the potential from the minimum potential VR ofthe reverse potential period 70 to the medium potential VM can beprevented, but the method of preventing accidental ejection of an inkdrop is not limited to this. For example, accidental ejection of an inkdrop can also be prevented by carrying out the increasing step 73 inaccordance with the period of oscillation of the meniscus. That is, theincreasing step 73 is carried out at a timing when the oscillation ofthe meniscus that has arisen due to the reducing step 71 of the reversepotential period 70 is at a stage at which the meniscus is being drawnin toward the pressure chamber 21 side. As a result, the oscillation ofthe meniscus arising due to the increasing step 73 and the oscillationof the meniscus that has arisen due to the reducing step 71 cancel oneanother out, and hence accidental ejection of an ink drop can beprevented.

In this way, accidental ejection of an ink drop can be prevented even ifthe gradient in the increasing step 73 of the reverse potential period70 is made to be relatively high, and hence yet faster driving can berealized.

12. TENTH EXAMPLE

FIG. 16 are graphs showing an example of the driving signal and thedisplacement of a piezoelectric element according to a tenth example ofthe present invention.

In the present example, as shown in FIG. 16A, the voltage of the reversepotential period 70 is selectively outputted between high potentialperiods 60, whereby two types of ink drop of different sizes to oneanother can be ejected.

Specifically, in the case that the voltage of the high potential period60 is continuously outputted with no intervening reverse potentialperiod 70, the displacement of the piezoelectric element 40 after eachhigh potential period 60 has been passed through becomes the mediumdisplacement DM′ as shown in FIG. 16B. As a result, the actualdisplacement amount dl of the piezoelectric element 40 due to thecontraction step 63 of the high potential period 60 becomes smaller thanthe displacement amount d2 in the case that the medium displacement DMis passed through, and hence the size of the ink drop ejected is smallerthan the size in the case that the medium displacement DM is passedthrough (the normal dot size).

Note, however, that the medium displacement DM′ after each highpotential period 60 is an approximately constant displacement. That is,in the case that the voltage of the high potential period 60 isoutputted in succession to the piezoelectric element 40, the size of theink drops ejected becomes smaller than the normal dot size, and yet thesize of the ink drops is approximately constant.

On the other hand, in the case that the voltage of a reverse potentialperiod 70 is outputted between high potential periods 60, the actualdisplacement amount d3 of the piezoelectric element 40 due to thecontraction step 63 of the high potential period 60 after the reversepotential period 70 is approximately the same as the displacement amountd2 in the case that the medium displacement DM is passed through, andhence an ink drop of the normal dot size is ejected.

Consequently, by selectively outputting the reverse potential period 70,two types of ink drop of different sizes to one another can easily beejected.

For example, by outputting high potential periods 60 and reversepotential periods 70 to the piezoelectric element 40, ink drops of thenormal dot size can be ejected. Moreover, by continuously outputtinghigh potential periods 60 with no intervening reverse potential periods70, ink drops of a small dot size can be ejected.

In this way, dot gradation control can be carried out merely bycontrolling the driving signal, and hence high-quality printing can berealized relatively easily.

In addition to an ink-ejecting head used in an ink jet recordingapparatus, the liquid-ejecting head driving method and liquid-ejectingapparatus of the present invention can also be applied to heads that jetout various liquids, for example heads that eject a liquid containing acolorant used in the manufacture of color filters for liquid crystaldisplays or the like, heads that eject a liquid containing an electrodematerial used in electrode formation for organic EL displays, FEDs(field emission displays) or the like, and heads that eject a liquidcontaining a biological organic substance used in biochip manufacture.

Industrial Applicability

According to the liquid-ejecting apparatus and the driving method of thepresent invention, a liquid-ejecting apparatus and a driving method canbe provided according to which targeted good characteristics can beobtained, and moreover the scope for material selection can bebroadened.

1. A liquid-ejecting apparatus that contracts a pressure chamber andthus ejects liquid through application of voltage to a piezoelectricbody; wherein a driving waveform applied to said piezoelectric bodyduring the liquid ejecting operation comprises a high potential periodin which a voltage exhibiting an electric field strength exceeding thecoercive electric field of said piezoelectric body is applied, and areverse potential period in which a voltage such that the potentialbecomes of the opposite polarity to the polarity in said high potentialperiod or the potential becomes zero is applied.
 2. The liquid-ejectingapparatus according to claim 1, wherein the voltage applied in saidreverse potential period is a voltage exhibiting an electric fieldstrength that does not exceed the coercive electric field of saidpiezoelectric body.
 3. The liquid-ejecting apparatus according to claim1, wherein the voltage applied in said reverse potential period is avoltage exhibiting an electric field strength of at least the coerciveelectric field of said piezoelectric body.
 4. The liquid-ejectingapparatus according to claim 3, wherein the voltage applied in saidreverse potential period is a voltage that eliminates residualpolarization of said piezoelectric body.
 5. The liquid-ejectingapparatus according to claim 3, wherein, after said pressure chamber hasexpanded in said reverse potential period, when contraction of saidpressure chamber has started, said pressure chamber is furthercontracted and hence said liquid is ejected in said high potentialperiod while said pressure chamber has not yet expanded again.
 6. Theliquid-ejecting apparatus according to claim 3, wherein, out of saidreverse potential period, the time period for which the voltageexhibiting an electric field strength of at least said coercive electricfield is applied is not more than 2 μs.
 7. The liquid-ejecting apparatusaccording to claim 3, wherein the time period between when the absolutevalue of the voltage applied in said reverse potential period starts todrop from a maximum value and when the absolute value of the voltageapplied in said high potential period reaches approximately a maximum isnot more than 2 μs.
 8. The liquid-ejecting apparatus according to anyone of claims 1 through 7, wherein the absolute value of the voltageapplied in said reverse potential period is not more than the absolutevalue of the maximum voltage in said high potential period.
 9. Theliquid-ejecting apparatus according to any one of claims 1 through 8,wherein the voltage is applied to a piezoelectric thin film.
 10. Theliquid-ejecting apparatus according to any one of claims 1 through 9,wherein a voltage exhibiting an electric field strength of at least1.5×10⁷ V/m is applied in said high potential period.
 11. Theliquid-ejecting apparatus according to any one of claims 1 through 10,wherein the voltage is applied at a frequency of at least 20 kHz. 12.The liquid-ejecting apparatus according to any one of claims 1 through11, wherein said driving waveform has one of said reverse potentialperiod per one of said high potential period.
 13. The liquid-ejectingapparatus according to any one of claims 1 through 12, wherein, out ofsaid driving waveform applied during the liquid ejecting operation, aportion corresponding to during a contraction operation of said pressurechamber contains at least part of said high potential period, and atleast part of said reverse potential period.
 14. The liquid-ejectingapparatus according to any one of claims 1 through 13, wherein thestrain in the piezoelectric body during liquid ejection by saidliquid-ejecting apparatus is at least 0.3%.
 15. The liquid-ejectingapparatus according to claim 1, wherein said driving waveform isconstituted such that the pressure chamber is contracted and henceliquid is ejected through a change in the potential from a prescribedmedium potential to the maximum potential in said high potential period,and the potential returns to said prescribed medium potential via saidreverse potential period, and moreover the change in the potential fromsaid reverse potential period to said prescribed medium potential ismade to have a gradient such that liquid is not ejected.
 16. Theliquid-ejecting apparatus according to claim 1 or 15, wherein thepotential is changed continuously from the maximum potential in saidhigh potential period to the potential in said reverse potential period.17. The liquid-ejecting apparatus according to claim 1, wherein thevoltage of said reverse potential period can be selectively applied perone application of the voltage of said high potential period.
 18. Aliquid-ejecting head driving method in which a pressure chamber iscontracted and thus liquid is ejected through application of voltage toa piezoelectric body; wherein a driving waveform applied to saidpiezoelectric body during a printing operation comprises a highpotential period in which a voltage exhibiting an electric fieldstrength exceeding the coercive electric field of said piezoelectricbody is applied, and a reverse potential period in which a voltage suchthat the potential becomes of the opposite polarity to the polarity insaid high potential period or the potential becomes zero is applied.